87 



The colour of some green insects is not altered either by muriatic 

 acid or carbonate of soda, and therefore appears to be a peculiar 

 principle differing from that of vegetables. 



Account of Experiments made on the Strength of Materials. By 

 George Rennie, jun. Esq. In a Letter to Thomas Young, M.D. 

 For. Sec. R.S. Read February 12, 1818. [Phil. Trans. 1818, 

 p. 118.] 



After taking a cursory view of the labours of others in this depart- 

 ment of mechanical inquiry, Mr. Rennie proceeds to give an account 

 of the apparatus which he employed, and of the result of his own 

 experiments. Of the resistances opposed to the simple strains which 

 may disturb the quiescent state of a body, the principal are : the re- 

 pulsive force, whereby it resists compression ; and the force of cohe- 

 sion, whereby it resists extension. On the former, with few excep- 

 tions, there is scarcely anything on record. Lagrange, in his Memoir 

 on the Force of Springs, published in 1760, represents the moment 

 of elasticity by a constant quantity, without indicating the relation 

 of this value to the size of the spring : but in the Memoir of 1770, on 

 the Forms of Columns, when he considers a body whose dimensions 

 and thickness are variable, he makes the moment of elasticity propor- 

 tional to the fourth power of the radius : but all these calculations, 

 says Mr. Rennie, are inapplicable to columns under common circum- 

 stances. The results of experiments are also extremely discordant ; 

 for it is deduced from those of Reynolds, that the power required to 

 crush a cubic quarter of an inch of cast iron is 200 tons, whereas in 

 the author's experiments upon cubes of the same size, the amount 

 never exceeded five tons ; and although Mr. Reynolds probably em- 

 ployed metal cast at the furnace of Maidley Wood, which is very 

 strong, yet this circumstance can have been but of little importance 

 compared with the great disproportion of results. 



Mr. Rennie employed four kinds of iron : the first taken from the 

 centre of a large block, similar in appearance to what is usually 

 called gun metal ; the second from a small casting, close-grained, and 

 of a dull gray colour ; the third, horizontally cast iron, in bars three 

 eighths of an inch square and eight inches long ; the fourth, similar 

 bars cast vertically. It appears from the annexed tables that the 

 vertical castings are stronger than those taken from the block. 



Some miscellaneous experiments relating to the different kinds of 

 wood and stone are also added to those on the metals. They show that 

 little dependence can be placed on the specific gravity of the stone ; 

 neither is hardness to be regarded as a characteristic of strength. In 

 the rupture of amorphous stones, Mr. Rennie remarks, that pyra- 

 mids are fonned, having for their base the upper side of the cube 

 next the lever, the action of which displaces the sides of the cubes 

 precisely as if a wedge had operated between them. 



